I could go either way.Never mind what you would prefer - do you believe that Schedule 4 makes the replacement of a 10.5kW shower by a 7.5kW one notifiable?
I would read MI of appliance, if the MI stated that 7.5kW shower should be on 32A MCB and protected by 30mA RCD.
And if either or both of those devices on the existing circuit supplying the shower needed to be either changed or installed.
I would notify.